This invention relates in general to the measurement of characteristics of a sample and in particular to a time-resolved measurement technique that excites a sample by means of a pump beam and then measures the sample characteristics by means of a probe beam.
Ellipsometry and reflectometry are powerful techniques for film thickness measurement in semiconductor processing. In cases where the film under examination is transparent to the illuminating radiation, ellipsometry for example can measure films down to one monolayer thick (3-10 Angstroms). However, both ellipsometry and reflectometry fail in cases where the film under examination is opaque. Metallic films, which play a major role in integrated circuit fabrication, fall into this category. Optical radiation is absorbed within the first few tens to hundreds of Angstroms of the film, depending on the wavelength. For example, using green radiation at a wavelength of 0.5 micron in aluminum, the absorption length is less than 83 Angstroms. At longer wavelengths, and in particular at infra red wavelengths, this situation gets better, but still ellipsometry cannot provide the full solution with reference to metallic films or other optically opaque films.
Time resolved pulse-echo ultrasound is a well known technique for thickness measurement in situations where the thickness of interest is a few millimeters or at least tens of microns. For films used in semiconductor processing, one needs extremely short pulses so that the surface echoes from subsurface film interfaces can be time resolved. Such pulses can be generated by ultrashort laser pulses and the general area of this art is known as picosecond photoacoustics. The physical processes involved are as follows: a short laser pulse is absorbed within one absorption length from the surface, causing a rise in local temperature of the surface. Through the temperature coefficient of expansion (expansivity) the film undergoes thermal stresses leading to a pulse, such as an elastic or acoustic pulse, which propagates across the film at the speed of sound. Given the velocity of sound in the film, if one measures the time of flight across the film, one can compute the film thickness. The key-remaining issue, is therefore, the detection of the acoustic disturbance once it bounces back from the rear side of the film and reaches the front surface.
A number of approaches have been proposed for measuring the thicknesses of films using the above-described time resolved pulse-echo technique. One approach is described in U.S. Pat. No. 6,108,087 assigned to the assignee of the present application. In this technique, a pump pulse is directed to the top surface of a film in a sample to generate an acoustic pulse. The acoustic pulse propagates downwards until it reaches an interface between the bottom surface of the film in the sample and the substrate or an other film and is reflected back to the top surface of the film as a first echo. A reflection of the first echo propagates downwards and is again reflected back towards the surface as the second echo. Interferometry is used to detect a displacement at the surface due to the arrival of the acoustic pulse, thereby leading to a measurement of the time lapse between the first and second echoes which, in turn, leads to the computation of the film thickness.
Another approach is described in U.S. Pat. No. 4,710,030.This patent states that once a stress pulse is reflected from the rear side of the film and reaches the surface, it changes the optical constants of the surface. The change in the optical constants of the surface leads to a change in reflectivity which is detected by monitoring the intensity of the reflection of a probe beam which also illuminates the surface.
In the approaches described above, however, the probe beams are measured at different time delays by means of a high resolution delay stage that is placed in the path of the pump beam or the probe beam. This delay stage requires that a roof mirror is mechanically translated in order to vary the relative timing relationship between the pump and probe beams so that the change in reflectivity or displacement of the surface of the sample can be measured at different relative time delays between the two beams in order to detect the echoes of the acoustic pulse caused by the pump pulse. In other words, after each measurement in which a sequence of pump pulses and the probe beam are applied to measure the reflectivity or displacement, the delay stage increases or decreases the relative delay between the pump and probe beams, and a next measurement is then taken. This is repeated until adequate data points have been acquired for identifying the times at which the first and the second echoes are detected. The time delay between the first and second echoes may then be used for determining film thickness. In order for adequate data points to be acquired, the above procedure requires a significant amount of time for repeating the measurements at different relative delays between the pump and probe beams.
It is therefore desirable to provide an improved technique for measuring film thickness(es) that are faster than the above-described approaches.